Method and system for selectively illuminated integrated photodetectors with configured launching and adaptive junction profile for bandwidth improvement

ABSTRACT

Methods and systems for selectively illuminated integrated photodetectors with configured launching and adaptive junction profile for bandwidth improvement may include a photonic chip comprising an input waveguide and a photodiode. The photodiode comprises an absorbing region with a p-doped region on a first side of the absorbing region and an n-doped region on a second side of the absorbing region. An optical signal is received in the absorbing region via the input waveguide, which is offset to one side of a center axis of the absorbing region; an electrical signal is generated based on the received optical signal. The first side of the absorbing region may be p-doped. P-doped and n-doped regions may alternate on the first and second sides of the absorbing region along the length of the photodiode. The absorbing region may comprise germanium, silicon, silicon/germanium, or similar material that absorbs light of a desired wavelength.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application claims priority to and the benefit of U.S. ProvisionalApplication 62/516,949 filed on Jun. 8, 2017, which is herebyincorporated herein by reference in its entirety.

FIELD

Aspects of the present disclosure relate to electronic components. Morespecifically, certain implementations of the present disclosure relateto methods and systems for selectively illuminated integratedphotodetectors with configured launching and adaptive junction profilefor bandwidth improvement.

BACKGROUND

Conventional approaches for integrated photodetectors may be costly,cumbersome, and/or inefficient—e.g., they may be complex and/or timeconsuming, and/or may have limited responsivity due to losses.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with some aspects of the present disclosureas set forth in the remainder of the present application with referenceto the drawings.

BRIEF SUMMARY

System and methods are provided for selectively illuminated integratedphotodetectors with configured launching and adaptive junction profilefor bandwidth improvement, substantially as shown in and/or described inconnection with at least one of the figures, as set forth morecompletely in the claims.

These and other advantages, aspects and novel features of the presentdisclosure, as well as details of an illustrated embodiment thereof,will be more fully understood from the following description anddrawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram of a photonically-enabled integrated circuitwith selectively illuminated integrated photodetectors, in accordancewith an example embodiment of the disclosure.

FIG. 1B is a diagram illustrating an exemplary photonically-enabledintegrated circuit, in accordance with an example embodiment of thedisclosure.

FIG. 1C is a diagram illustrating a photonically-enabled integratedcircuit coupled to an optical fiber cable, in accordance with an exampleembodiment of the disclosure.

FIGS. 2A and 2B illustrate two examples of integrated photodetectors, inaccordance with an example embodiment of the disclosure.

FIGS. 3A and 3B illustrate calculated bandwidths for various portions ofa germanium photodiode, in accordance with an example embodiment of thedisclosure.

FIG. 4 illustrates top and cross-section views of a photodiode withasymmetric launching, in accordance with an example embodiment of thedisclosure.

FIG. 5 shows the top view power profile of a photodiode with asymmetriclaunching, in accordance with an example embodiment of the disclosure.

FIG. 6 illustrates a photodiode with evanescent coupling, in accordancewith an example embodiment of the disclosure.

FIG. 7 illustrates a photodiode with alternating n-type and p-typedoping, in accordance with an example embodiment of the disclosure.

FIG. 8 illustrates bandwidth profiles along a variable junction locationgermanium photodiode, in accordance with an example embodiment of thedisclosure.

DETAILED DESCRIPTION

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprise afirst “circuit” when executing a first one or more lines of code and maycomprise a second “circuit” when executing a second one or more lines ofcode. As utilized herein, “and/or” means any one or more of the items inthe list joined by “and/or”. As an example, “x and/or y” means anyelement of the three-element set {(x), (y), (x, y)}. In other words, “xand/or y” means “one or both of x and y”. As another example, “x, y,and/or z” means any element of the seven-element set {(x), (y), (z), (x,y), (x, z), (y, z), (x, y, z)}. In other words, “x, y and/or z” means“one or more of x, y and z”. As utilized herein, the term “exemplary”means serving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.,” and “for example” set off lists ofone or more non-limiting examples, instances, or illustrations. Asutilized herein, circuitry or a device is “operable” to perform afunction whenever the circuitry or device comprises the necessaryhardware and code (if any is necessary) to perform the function,regardless of whether performance of the function is disabled or notenabled (e.g., by a user-configurable setting, factory trim, etc.).

FIG. 1A is a block diagram of a photonically-enabled integrated circuitwith selectively illuminated integrated photodetectors, in accordancewith an example embodiment of the disclosure. Referring to FIG. 1A,there is shown optoelectronic devices on a photonically-enabledintegrated circuit 130 comprising optical modulators 105A-105D,photodiodes 111A-111D, monitor photodiodes 113A-113H, and opticaldevices comprising couplers 103A-103K, optical terminations 115A-115D,grating couplers 117A-117H, and mode converters 121. There are alsoshown electrical devices and circuits comprising amplifiers 107A-107D,analog and digital control circuits 109, and control sections 112A-112D.The amplifiers 107A-107D may comprise transimpedance and limitingamplifiers (TIA/LAs), for example.

In an example scenario, the photonically-enabled integrated circuit 130comprises a complementary metal oxide semiconductor (CMOS) photonics diewith a laser assembly 101 coupled to the top surface of the IC 130. Thelaser assembly 101 may comprise one or more semiconductor lasers withisolators, lenses, and/or rotators for directing one or more CW opticalsignals to the coupler 103A. The photonically enabled integrated circuit130 may comprise a single chip, or may be integrated on a plurality ofdie, such as one or more electronics die and one or more photonics die.

Optical signals are communicated between optical and optoelectronicdevices via optical waveguides 110 fabricated in thephotonically-enabled integrated circuit 130. Single-mode or multi-modewaveguides may be used in photonic integrated circuits. Single-modeoperation enables direct connection to optical signal processing andnetworking elements. The term “single-mode” may be used for waveguidesthat support a single mode for each of the two polarizations,transverse-electric (TE) and transverse-magnetic (TM), or for waveguidesthat are truly single mode and only support one mode whose polarizationis TE, which comprises an electric field parallel to the substratesupporting the waveguides. Two typical waveguide cross-sections that areutilized comprise strip waveguides and rib waveguides. Strip waveguidestypically comprise a rectangular cross-section, whereas rib waveguidescomprise a rib section on top of a waveguide slab. Of course, otherwaveguide cross section types are also contemplated and are within thescope of the disclosure.

In an example scenario, the couplers 103A-103C may comprise low-lossY-junction power splitters where coupler 103A receives an optical signalfrom the laser assembly 101 and splits the signal to two branches thatdirect the optical signals to the couplers 103B and 103C, which splitthe optical signal once more, resulting in four roughly equal poweroptical signals.

The optical modulators 105A-105D comprise Mach-Zehnder or ringmodulators, for example, and enable the modulation of thecontinuous-wave (CW) laser input signal. The optical modulators105A-105D may comprise high-speed and low-speed phase modulationsections and are controlled by the control sections 112A-112D. Thehigh-speed phase modulation section of the optical modulators 105A-105Dmay modulate a CW light source signal with a data signal. The low-speedphase modulation section of the optical modulators 105A-105D maycompensate for slowly varying phase factors such as those induced bymismatch between the waveguides, waveguide temperature, or waveguidestress. Compensating for these slowly varying phase factors is referredto as the passive phase, or the passive biasing of the Mach-ZehnderModulator (MZM).

The outputs of the optical modulators 105A-105D may be optically coupledvia the waveguides 110 to the grating couplers 117E-117H. The couplers103D-103K may comprise four-port optical couplers, for example, and maybe utilized to sample or split the optical signals generated by theoptical modulators 105A-105D, with the sampled signals being measured bythe monitor photodiodes 113A-113H. The unused branches of thedirectional couplers 103D-103K may be terminated by optical terminations115A-115D to avoid back reflections of unwanted signals.

The grating couplers 117A-117H comprise optical gratings that enablecoupling of light into and out of the photonically-enabled integratedcircuit 130. The grating couplers 117A-117D may be utilized to couplelight received from optical fibers into the photonically-enabledintegrated circuit 130, and the grating couplers 117E-117H may beutilized to couple light from the photonically-enabled integratedcircuit 130 into optical fibers. The grating couplers 117A-117H maycomprise single polarization grating couplers (SPGC) and/or polarizationsplitting grating couplers (PSGC). In instances where a PSGC isutilized, two input, or output, waveguides may be utilized.

Grating couplers are devices in integrated optical circuits thatinterface light between telecommunication fibers and optical circuits.They comprise surface emitting elements that diffract guided light outof the plane of the circuit, where it can be collected with standardoptical fibers. In contrast to other coupling methods, such as end-facecoupling, grating couplers lend themselves to planar fabrication methodsand allow free placement of optical interfaces on the chip surface.

In another exemplary embodiment illustrated in FIG. 1B, optical signalsmay be communicated directly into the surface of thephotonically-enabled integrated circuit 130 without optical fibers bydirecting a light source on an optical coupling device in the chip, suchas the light source interface 135 and/or the optical fiber interface139. This may be accomplished with directed laser sources and/or opticalsources on another chip flip-chip bonded to the photonically-enabledintegrated circuit 130.

The photodiodes 111A-111D may convert optical signals received from thegrating couplers 117A-117D into electrical signals that are communicatedto the amplifiers 107A-107D for processing. In another embodiment of thedisclosure, the photodiodes 111A-111D may comprise high-speedheterojunction phototransistors, for example, and may comprise germanium(Ge) in the absorbing region for absorption in the 1.3-1.6 μm opticalwavelength range, and may be integrated on a CMOS silicon-on-insulator(SOI) wafer, for example.

As the speeds of optoelectronic transmitters increase, photodetectorspeed must be improved for overall system performance. In an exampleembodiment of the disclosure, selectively illuminated integratedphotodetectors with launching and adaptive junction profile forbandwidth improvement are described for receiving optical signals from aplurality of waveguides, as shown further with respect to FIGS. 2-8.

In an example embodiment of the disclosure, integrated photodiodebandwidth may be improved by selectively illuminating only the highspeed portion of the absorbing active core. This can be achieved usingan asymmetric light launching condition. In addition, bandwidth may befurther improved using an adaptive doping/junction profile to ensurethat the launched light overlaps only with the high speed portion of thedetector along the propagation direction.

In another example embodiment, the four transceivers shown in FIG. 1Amay be incorporated in two or more chips, as opposed to the single chipshown. For example, the electronics devices, such as the controlcircuits 109 and the amplifiers/TIAs 107A-107D, may be fabricated in anelectronics CMOS die while the optical and optoelectronic devices, suchas the photodetectors 111A-111D, grating couplers 117A-117H, and theoptical modulators 105A-105D may be incorporated on a photonics die,such as a silicon photonics interposer.

The analog and digital control circuits 109 may control gain levels orother parameters in the operation of the amplifiers 107A-107D, which maythen communicate electrical signals off the photonically-enabledintegrated circuit 130. The control sections 112A-112D compriseelectronic circuitry that enable modulation of the CW laser signalreceived from the splitters 103A-103C. The optical modulators 105A-105Dmay require high-speed electrical signals to modulate the refractiveindex in respective branches of a MZM, for example.

In operation, the photonically-enabled integrated circuit 130 may beoperable to transmit and/or receive and process optical signals. Opticalsignals may be received from optical fibers by the grating couplers117A-117D and converted to electrical signals by the photodetectors111A-111D. The electrical signals may be amplified by transimpedanceamplifiers in the amplifiers 107A-107D, for example, and subsequentlycommunicated to other electronic circuitry, not shown, in thephotonically-enabled integrated circuit 130.

Integrated photonics platforms allow the full functionality of anoptical transceiver to be integrated on a single chip. An opticaltransceiver chip contains optoelectronic circuits that create andprocess the optical/electrical signals on the transmitter (Tx) and thereceiver (Rx) sides, as well as optical interfaces that couple theoptical signals to and from a fiber. The signal processing functionalitymay include modulating the optical carrier, detecting the opticalsignal, splitting or combining data streams, and multiplexing ordemultiplexing data on carriers with different wavelengths.

Typically, the p-i-n junction of a waveguide photodiode is orientated ina lateral direction and the intrinsic width, which is usually the widthof the absorbing region, is defined by photolithography. This type ofdesign is commonly used because of its fabrication simplicity. Althoughthe length of the photodiode is short and therefore results in a lowercapacitance, the intrinsic width of the absorbing region is limited to˜500 nm due to lithography issues, which means that this type ofjunction photodiode is transit-time-limited. On the other hand, avertical p-i-n photodiode is limited by the absorbing layer thickness,which is determined by the deposition process, and can be easily go downbelow 500 nm. The bandwidth can then be higher and nottransmit-time-limited. However, a vertical junction utilizes a morecomplex contact scheme which either (1) increases fabrication complexityor (2) contact resistance. Other design options are to vary the dopingprofile to reduce the effective Ge intrinsic width, e.g. making thep-i-n junction more vertical-like. In an example scenario, the absorbingregion comprises germanium, but may comprise other materials such assilicon, silicon-germanium, or other semiconductor that absorbs light atthe desired wavelength.

In an example embodiment of the disclosure, an alternative method whichis an optical approach, is described to increase bandwidth withoutadding process complexity to the existing process technology. Examplesof the proposed approach applied to actual fabricated devices are shownin FIGS. 2-8.

FIG. 1B is a diagram illustrating an exemplary photonically-enabledintegrated circuit, in accordance with an example embodiment of thedisclosure. Referring to FIG. 1B, there is shown thephotonically-enabled integrated circuit 130 comprising electronicdevices/circuits 131, optical and optoelectronic devices 133, a lightsource interface 135, a chip front surface 137, an optical fiberinterface 139, CMOS guard ring 141, and a surface-illuminated monitorphotodiode 143.

The light source interface 135 and the optical fiber interface 139comprise grating couplers, for example, that enable coupling of lightsignals via the CMOS chip surface 137. Coupling light signals via thechip surface 137 enables the use of the CMOS guard ring 141 whichprotects the chip mechanically and prevents the entry of contaminantsvia the chip edge.

The electronic devices/circuits 131 comprise circuitry such as theamplifiers 107A-107D and the analog and digital control circuits 109described with respect to FIG. 1A, for example. The optical andoptoelectronic devices 133 comprise devices such as the couplers103A-103K, optical terminations 115A-115D, grating couplers 117A-117H,optical modulators 105A-105D, high-speed heterojunction photodiodes111A-111D, and monitor photodiodes 113A-113I.

FIG. 1C is a diagram illustrating a photonically-enabled integratedcircuit coupled to an optical fiber cable, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 1C, there is shown thephotonically-enabled integrated circuit 130 comprising the chip surface137, and the CMOS guard ring 141. There is also shown a fiber-to-chipcoupler 145, an optical fiber cable 149, and an optical source assembly147.

The photonically-enabled integrated circuit 130 comprises the electronicdevices/circuits 131, the optical and optoelectronic devices 133, thelight source interface 135, the chip surface 137, and the CMOS guardring 141 may be as described with respect to FIG. 1B, for example.

In an example embodiment, the optical fiber cable may be affixed, viaepoxy for example, to the CMOS chip surface 137. The fiber chip coupler145 enables the physical coupling of the optical fiber cable 149 to thephotonically-enabled integrated circuit 130. In another examplescenario, the IC 130 may comprise photonic devices on one die, such as aphotonics interposer, and electrical devices on an electronics die, bothof which may comprise CMOS die.

FIGS. 2A and 2B illustrate two examples of integrated photodetectors, inaccordance with an example embodiment of the disclosure. An examplestandard waveguide photodetector is based on an absorbing germanium (Ge)layer grown on a silicon waveguide, although other materials arepossible. Referring to FIG. 2A, there is shown integrated photodiode 200comprising oxide layer 201, which is on a silicon layer (not shown) in atypical SOI wafer, p+ region 203, intrinsic region 205, n+ region 207,absorbing region 209, and trench 211.

The intrinsic width of the photodiode 200 in this case is the width ofthe absorbing region 209. A typical p-i-n integrated photodiode issymmetric, as shown in FIG. 2A, except the doping type is different onthe right (n+ region 207) and left (p+ region 203) sides. For atransit-time-limited device, the bandwidth is limited by the travellingtime of electrons or holes, whichever are slower from the absorbingregion in the absorbing region 209 to the contacts at the high doped p+and n+ regions. In instances where Ge is used for the absorbing region209, the mobility of electrons is much higher than holes in bulk Ge.Similarly for Ge-on-Si, the bandwidth is limited by holes, so holes thatare generated near the n-doped side (right side) of the diode thereforetake the longest time to be swept out from right to left, while highermobility electrons are more quickly swept out to the n+ region 207. Thissuggests that carriers generated at different parts of the absorbingcore can be swept out quite differently. This same concept applies forother materials where holes and electrons have different mobility.

FIG. 2B illustrates a vertical-like photodiode 250 comprising oxidelayer 201, p+ region 203, n+ region 207, absorbing region 209, trench211, and p+ region 213. In this structure, the absorbing region 209 isalso undoped except in the upper left corner and left edge, in the p+region 213, meaning photo-generated carriers may be swept out in anessentially vertical direction, where the absorbing region 209 thicknesscan be very thin, determined by the deposition technique. In thismanner, carriers generated in the absorbing region 209 merely need to beswept downward through the thickness of the absorbing region 209.

In an example scenario, the diodes shown in FIGS. 2A and 2B may comprisedouble heterostructures where the p+ and n+ regions are silicon and theabsorbing region 209 comprises germanium, although the disclosure is notso limited as other junctions are possible, such as homojunctions, asingle heterojunction, or multiple junctions. Accordingly, the n-doped,p-doped regions, and absorbing region 209 may all comprise Ge or Si (orother suitable material) for a homojunction, or Si and Ge for aheterojunction, for example. Furthermore, while germanium or silicon isshown in this example for the absorbing region 209, silicon-germanium orother materials that absorb optical signals at a desired wavelength maybe utilized.

FIGS. 3A and 3B illustrate calculated bandwidths for various portions ofa germanium photodiode, in accordance with an example embodiment of thedisclosure. The bandwidth indicated in each portion is the bandwidth ofthe detector assuming light is only absorbed on that portion. FIG. 3Ashows the undoped absorbing region divided into four portions laterallyfor the lateral p-i-n photodetector 200 of FIG. 2A. The center twoportions have a much higher bandwidth but the overall device speed, whenuniformly illuminated, is limited by the time required to sweep outcarriers near the n-doped side, due to the slower holes, resulting in anoverall bandwidth of the device of only 32 GHz. In a vertical-like p-i-nintegrated photodiode as shown in FIG. 2B, and bandwidth in FIG. 3B, thesituation is different owing to the quasi-vertical orientation of theelectric field.

FIG. 3B shows the bandwidth of each portion of the absorbing region fora vertical-like photodiode as shown by photodetector 250 in FIG. 2B. Inthis example, the absorbing region is germanium and the contact layersare silicon, although other materials are possible for this structure.The left two portions of the device have a much higher bandwidth of 76GHz and 95 GHz, respectively, whereas the overall bandwidth of thedevice is only 23 GHz if fully illuminated. Therefore, by controllingwhere the optical signal is launched and absorbed into the photodiode,speed may be greatly increased as compared to a device where the lightimpinges on the entire absorbing structure. While a Ge—Siheterostructure is use for this example, other materials and structuresmay be utilized.

Asymmetric launching for increased bandwidth comprises illuminatinglight only onto the high speed portions. To achieve this, a waveguideoffset may be utilized to direct light straight into the selectedabsorbing region where speed is highest, as compared to a conventionalwaveguide which is aligned to the center of the diode. Since light isstrongly absorbed by Ge, for example, the waveguide offset does notgreatly affect the responsivity. Depending on the optical propagationand absorption length, the bandwidth may be improved without adding anyprocess complexity. For example, the bandwidth achievable byilluminating the left two portions of Ge of a vertical-like p-i-nphotodiode shown in FIG. 3B is very high, 76 and 95 GHz. Therefore, theinput waveguide may be offset from the center and directly coupled tothe left side (p-doped side) of the diode to gain speed.

FIG. 4 illustrates top and cross-section views of a photodiode withasymmetric launching, in accordance with an example embodiment of thedisclosure. Referring to FIG. 4, there is shown photodetector 400comprising oxide layer 401, p+ region 403, n+ region 407, absorbingregion 409, trench 411, p+ region 413, and waveguide 415. The waveguide415 comprises the portion of a Si layer 405, for example, although othermaterials are possible, between the trenches 411. In addition, in thisexample, the absorbing region 409 may comprise Ge, although othermaterials are of course possible. The trenches 411 provide an index ofrefraction change from the Si layer 405 so that optical signals may beconfined in the waveguide 415. The trenches 411 comprise regions wherethe Si layer 405 has been removed and either filled with oxide or otherdielectric, or left unfilled for an air/silicon index of refractiondifference.

The dashed lines indicate where in the upper view the cross-sectionalviews below correspond. Accordingly, the lower left view shows a crosssectional view of the waveguide 415 and the lower right view shows across-sectional view of the photodetector 400. The lower left portion ofFIG. 4 illustrates a cross-section of the input waveguide with a widertrench 411 on the right side as compared to the left, resulting in theinput waveguide 415 being offset to the left of center (or above centerin the top view) of the Ge absorbing layer, as indicated by theWaveguide Offset.

The light incident on the absorbing region 409 from a Si single modewaveguide 415 is expected to spread out in a short distance, asindicated by the expanding optical mode in the absorbing region 409.However, in an example scenario where Ge is used as the absorbing region409, a large portion of the light (˜95%) is absorbed in Ge within thefirst 5 μm of propagation distance.

The lower right view of FIG. 4 shows a cross section of the photodiodewith a vertical-like p-i-n structure, as shown in FIG. 2B, for example.Since the bandwidth is highest in the left two quadrants of theabsorbing layer 409 for this structure, as shown in FIG. 3B, the inputwaveguide 415 is offset to align to this higher bandwidth portion,increasing the overall bandwidth of the photodiode.

In this manner, the bandwidth of the photodiode 400 may be configured bythe waveguide offset, where different doping configurations may resultin different locations of maximum bandwidth in the absorbing region 409.While double heterostructure diodes are shown in FIG. 4, the disclosureis not so limited as other junctions are possible, such ashomojunctions, a single heterojunction, or multiple junctions.Furthermore, while germanium or silicon is shown in this example for theabsorbing region 409, silicon-germanium or other materials that absorboptical signals at a desired wavelength may be utilized.

FIG. 5 shows the top view optical power profile of a photodiode withasymmetric launching, in accordance with an example embodiment of thedisclosure. Referring to FIG. 5, there is shown an input waveguide 515,a wider waveguide section 515A, and an absorbing region 509. There isalso shown optical power profile 517 in the absorbing region 509, wherethe profile is shown for light being incident on the absorbing region509 with a waveguide offset oriented above the center of the absorbingregion 509, which in this example comprises Ge. This launching conditionshows that light is actually bouncing back and forth in the absorbingregion 509 and optical power stays at one side of the absorbing region509 and then the other side half in every period. In an examplescenario, where the p-i-n photodiode comprises a Si—Ge doubleheterostructure, this periodic pattern is the result of beating of thedifferent modes excited in the Ge/Si waveguide by the asymmetriclaunching condition. The actual phase of the beating pattern can becontrolled by an appropriate launching condition

Maximum benefit for the bandwidth can be achieved if the light stays aslong as possible, in propagation, on the fast side of the device. Infact, where Ge is used for the absorbing region 509, since theabsorption of Ge is very high, after the first half beating period mostof the light might be already absorbed. Therefore controlling the phaseof the beating pattern is key to maximize the bandwidth enhancementbenefit. The phase of the beating pattern can be adjusted either byvarying the waveguide launching angle or by adding a wider waveguide,the wider waveguide section 515A between the input waveguide 515 and theabsorbing region 509, as shown in FIG. 5. In this case, most of thelight absorbed in the first half section stays on the absorbing portionwith higher bandwidth, i.e., the upper two quadrants of the widerwaveguide section 515A, which improves the overall bandwidth.

FIG. 6 illustrates a photodiode with evanescent coupling, in accordancewith an example embodiment of the disclosure. Referring to FIG. 6, thereis shown photodetector 600 comprising a p+ region 603, n+ region 607,absorbing region 609, and input waveguide 615. There is also shownoptical mode 617 in the waveguide 615 after reaching the photodetector600, with an evanescent portion 619 that extends into the absorbingregion 609. In an example scenario, the absorbing region comprises Geand the p+ and n+ regions comprise silicon, although other materials arepossible.

Evanescent coupling is another technique to achieve an asymmetriclaunching condition where the input optical wave propagates in a veryclose parallel waveguide 615, as shown in FIG. 6. Optical modes are notcompletely confined to waveguides in which they are traveling, with aportion of the signal comprising evanescent waves that extend beyond thewaveguide edge. In this case, the waveguide 615 may be highly doped tomaintain good contact resistance but the propagation length is short aslight may be strongly absorbed by Ge with a strong evanescent couplingstructure. Furthermore, while germanium or silicon is shown in thisexample for the absorbing region 509, silicon-germanium or othermaterials that absorb optical signals at a desired wavelength may beutilized.

FIG. 7 illustrates an integrated photodiode with alternating n-type andp-type doping, in accordance with an example embodiment of thedisclosure. Referring to FIG. 7, there is shown top and cross-sectionalviews of photodetector 700 comprising oxide layer 701, alternating p+regions 703A and 703B, alternating n+ regions 707A and 707B, absorbingregion 709, and alternating absorbing p+ regions 713A and 713B. The topview shows the optical power profile 717 as it proceeds through theabsorbing region 709 on one side and then the other half, in everyperiod. In an example scenario, the absorbing region 709 comprises Ge.In this case, the light coupled in the absorbing region 709 shows abeating pattern with the power confined in a much narrower width thanthe absorbing region 709 width. The doping profile may be varied and/oralternated along the propagation distance such that the high speed Geportions, in this example, always overlap with the area where highoptical power is localized. This adaptive junction profile can beachieved by a) flipping the n and p side every half of the beatingperiod, and/or b) varying the doping profile along with the beatingpattern.

The lower cross-sectional views, indicated by the arrows from differentboxed regions, shows this flipping of the n and p side every halfbeating period so that the optical power is always at the higher speedportion of the absorbing region 709 (e.g., the p-doped side). Thisconfiguration might have higher parasitic capacitance but only one ortwo periods may be needed due to the strong absorption of the absorbingregion 709, such as when Ge or similar material is utilized, minimizingthis disadvantage. While double heterostructure diodes are shown in FIG.7, the disclosure is not so limited as other junctions are possible,such as homojunctions, a single heterojunction, or multiple junctions.Accordingly, the n-doped and p-doped regions alongside the Ge absorbinglayer may comprise Ge for a homojunction, or Si for a heterojunction,for example.

FIG. 8 illustrates bandwidth profiles along a variable junction locationphotodiode, in accordance with an example embodiment of the disclosure.Referring to the top portion of FIG. 8, there is shown a top view of aphotodiode where the width of the p-type and n-type silicon layersvaries linearly in a zig-zag pattern to place the junction such that thehigh speed portion corresponds to the optical mode in the asymmetricallylaunched photodiode.

The lower portions of FIG. 8 illustrate the bandwidth for the photodiodein three locations a, b, and c, indicated by the dashed lines in theupper view, along the length where the PN junction location is changed.In an example scenario, Ge may be used for the absorbing region,although other materials are possible. The results show that the twoabsorbing portions near the PN junction have higher bandwidth than theother two. Therefore, by varying the junction location or doping profilesuch that it follows the bouncing path of the optical mode, the mostcarriers are generated in the higher speed portion along the propagationdistance. This adaptive approach can be easily implemented by devicelayout. While a double heterostructure diode with Ge absorbing regionand Si contact layers is shown in FIG. 8, the disclosure is not solimited as other junctions are possible, such as homojunctions, a singleheterojunction, or multiple junctions. Accordingly, the n-doped andp-doped regions alongside the Ge absorbing layer may comprise Ge for ahomojunction, or Si for a heterojunction, for example.

In an example embodiment of the disclosure, a method and system isdescribed for selectively illuminated integrated photodetectors withlaunching and adaptive junction profile for bandwidth improvement. Themethod may comprise, in a photonic chip comprising an input waveguideand a photodiode, where the photodiode comprises an absorbing regionwith a p-doped region on a first side of the absorbing region and ann-doped region on a second side of the absorbing region: receiving anoptical signal in the absorbing region via the input waveguide, which isoffset to one side of a center axis of the absorbing region; andgenerating an electrical signal based on the received optical signal.

The first side of the absorbing region may be p-doped. The p-dopedregion on the first side of the absorbing region may be followed by ann-doped region on the first side along a length of the photodiode andthe n-doped region on the second side of the absorbing region may befollowed by a p-doped region along the length of the photodiode. Thealternating p-doped and n-doped regions may repeat along the length ofthe photodiode. A junction at an interface between the p-doped layer andthe n-doped region may form a zig-zag pattern along a length of thephotodiode. A second waveguide may be between the absorbing region andthe input waveguide and has a larger width than the input waveguide. Theabsorbing region may comprise germanium, silicon, silicon-germanium, orother semiconductor that absorbs light at a desired wavelength.

An optical power of the received optical signal may be at a maximum in across-section of the germanium layer near a junction formed by then-doped region and the p-doped region. A location of maximum opticalpower in cross-sections of the photodiode may vary along a length of thephotodiode. The optical signal may be received from the input waveguideas evanescent waves when the input waveguide is offset beyond an outeredge of the absorbing region. The photonic chip may comprise acomplementary metal oxide semiconductor (CMOS) chip. The p-doped regionand the n-doped region may comprise silicon, germanium, or othersuitable semiconductor.

In another example embodiment of the disclosure, a method and system isdescribed for selectively illuminated integrated photodetectors withconfigured launching and adaptive junction profile for bandwidthimprovement. The system may comprise a photonic chip comprising an inputwaveguide and a semiconductor photodiode, where the semiconductorphotodiode comprises a semiconductor absorbing region with a p-dopedregion on a first side of the semiconductor absorbing region and ann-doped region on a second side of the absorbing region. Thesemiconductor absorbing region may comprise silicon, germanium,silicon-germanium, or other material that absorbs optical signals of thedesired wavelength. The photonic chip is operable to: receive an opticalsignal in the absorbing region via the input waveguide, which is offsetto one side of a center axis of the absorbing region; and generate anelectrical signal based on the received optical signal.

While the present disclosure has been described with reference tocertain embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the scope of the present invention. In addition,many modifications may be made to adapt a particular situation ormaterial to the teachings of the present invention without departingfrom its scope. Therefore, it is intended that the present invention notbe limited to the particular embodiment disclosed, but that the presentinvention will include all embodiments falling within the scope of theappended claims.

What is claimed is:
 1. A method for communication comprising:propagating an optical signal along a first center axis of an inputwaveguide of a photonic chip, the first center axis extending along afirst dimension; receiving the optical signal in an absorbing region ofa photodiode, the absorbing region having a second center axis extendingalong the first dimension, wherein the first center axis is offset fromthe second center axis along a second dimension different than the firstdimension, wherein the photodiode further comprises a semiconductorlayer arranged beneath the absorbing region and extending to opposingfirst and second sides of the absorbing region relative to the seconddimension, wherein the semiconductor layer comprises one or more p-dopedregions and one or more n-doped regions arranged such that, along anextent of the semiconductor layer in the first dimension, a location ofa P-N junction of the semiconductor layer within the second dimensionvaries; and generating an electrical signal based on the receivedoptical signal.
 2. The method according to claim 1, wherein the firstside of the absorbing region is p-doped.
 3. The method according toclaim 1, wherein the semiconductor layer comprises: a first plurality ofdoped regions extending to the first side of the absorbing region,wherein a first p-doped region of the one or more p-doped regions isfollowed in the first dimension by a first n-doped region of the one ormore n-doped regions; and a second plurality of doped regions extendingto the second side of the absorbing region, wherein a second n-dopedregion of the one or more n-doped regions is followed in the firstdimension by a second p-doped region of the one or more p-doped regions.4. The method according to claim 3, wherein each of the first pluralityof doped regions and the second plurality of doped regions comprisesalternating p-doped and n-doped regions that repeat along the firstdimension.
 5. The method according to claim 1, wherein the P-N junctionforms a zig-zag pattern along the first dimension.
 6. The methodaccording to claim 1, wherein the photonic chip further comprises: asecond waveguide arranged between the absorbing region and the inputwaveguide, wherein the second waveguide has a larger width in the seconddimension than the input waveguide.
 7. The method according to claim 1,wherein an optical power of the received optical signal is at a maximumin a cross-section of the semiconductor layer near the P-N junction. 8.The method according to claim 7, wherein a location of maximum opticalpower in cross-sections of the photodiode varies along a length of thephotodiode in the first dimension.
 9. The method according to claim 1,wherein receiving the optical signal comprises receiving evanescentwaves when the input waveguide is offset beyond an outer edge of theabsorbing region.
 10. The method according to claim 1, wherein theabsorbing region comprises germanium, and wherein the p-doped region andthe n-doped region comprise one or both of silicon and germanium.
 11. Asystem for communication, the system comprising: a photonic chipcomprising: an input waveguide configured to propagate an optical signalalong a first center axis, the first center axis extending along a firstdimension; and a photodiode comprising: an absorbing region configuredto receive the optical signal, the absorbing region having a secondcenter axis extending along the first dimension, wherein the firstcenter axis is offset from the second center axis along a seconddimension different than the first dimension; and a semiconductor layerarranged beneath the absorbing region and extending to opposing firstand second sides of the absorbing region relative to the seconddimension, wherein the semiconductor layer comprises one or more p-dopedregions and one or more n-doped regions arranged such that, along anextent of the semiconductor layer in the first dimension, a location ofa P-N junction of the semiconductor layer within the second dimensionvaries.
 12. The system according to claim 11, wherein the first side ofthe absorbing region is p-doped.
 13. The system according to claim 11,wherein the semiconductor layer comprises: a first plurality of dopedregions extending to the first side of the absorbing region, wherein afirst p-doped region of the one or more p-doped regions is followed inthe first dimension by a first n-doped region of the one or more n-dopedregions; and a second plurality of doped regions extending to the secondside of the absorbing region, wherein a second n-doped region of the oneor more n-doped regions is followed in the first dimension by a secondp-doped region of the one or more p-doped regions.
 14. The systemaccording to claim 13, wherein each of the first plurality of dopedregions and the second plurality of doped regions comprises alternatingp-doped and n-doped regions that repeat along the first dimension. 15.The system according to claim 11, wherein the P-N junction forms azig-zag pattern along the first dimension.
 16. The system according toclaim 11, wherein the photonic chip further comprises: a secondwaveguide arranged between the absorbing region and the input waveguide,wherein the second waveguide has a larger width in the second dimensionthan the input waveguide.
 17. The system according to claim 11, whereinan optical power of the received optical signal is at a maximum in across-section of the semiconductor layer near the P-N junction.
 18. Thesystem according to claim 17, wherein a location of maximum opticalpower in cross-sections of the photodiode varies along a length of thephotodiode in the first dimension.
 19. The system according to claim 11,wherein the photonic chip is operable to receive the optical signal fromthe input waveguide as evanescent waves when the input waveguide isoffset beyond an outer edge of the absorbing region.
 20. A method forcommunication comprising: receiving, from an input waveguide having afirst center axis extending along a first dimension, an optical signalin an absorbing region of a semiconductor photodiode, the absorbingregion having a second center axis extending along the first dimension,wherein the first center axis is offset from the second center axisalong a second dimension different than the first dimension, wherein thesemiconductor photodiode further comprises a semiconductor layerarranged beneath the absorbing region and extending to opposing firstand second sides of the absorbing region relative to the seconddimension, wherein the semiconductor layer comprises one or more p-dopedregions and one or more n-doped regions arranged such that, along anextent of the semiconductor layer in the first dimension, a location ofa P-N junction of the semiconductor layer within the second dimensionvaries; and generating an electrical signal based on the receivedoptical signal.